1. Field
The invention relates to an ALD-deposited material, suitable for use in optical coatings. In particular, the invention relates to a material having alternating layers for providing a high refractive index and low optical loss, to a method for producing said material, and to the use thereof in optical structures.
2. Description of Related Art
Optical coatings are used in optical devices to provide various final properties. Such coatings include filters, antireflective coatings and reflectors. These coatings are typically composed of layers of different materials varying in both thickness and refractive properties, in order to produce the required spectral response. “Optical” in this context refers to electromagnetic wavelengths in the range 180 nm to 20 μm.
Optical losses (absorption and scattering loss), refractive index and film stress or tension are very important parameters in the field of thin film optical coatings. The absence of point defects, like metallic impurities, is also especially important for applications using laser light.
Additionally, most applications require high performance in environmental tests, demanding as small effects from humidity and temperature variations as possible. In the typical design case, optical losses should be as small as possible, and film layers with as high and as low refractive index as possible should be available. A coating design typically comprises more than one thin film material, and a plurality of layers. Complex optical coatings may contain several hundred alternating layers of high and low refractive index. The thickness of the layers also varies according to the required properties. Such optical structures are usually designed using dedicated computer programs.
Amorphous films typically cause less loss than crystalline films, in which various interaction effects occur, causing scattering and absorption. Optical losses in thin films containing crystals typically depend on the crystal size and on the light wavelength, so that less loss occurs at longer wavelengths than at shorter wavelengths. The importance of losses naturally depends on the application, but generally speaking low losses enable a wider range of designs or provide performance benefits.
Materials having a high refractive index are advantageous. The designs perform better, and the manufacturing of actual devices is typically easier and less costly, as the total film thickness is reduced.
Optical losses are often measured using a spectrophotometer. Due to optical interference, film transmission depends on the film thickness, but the effects of optical interference are not relevant at maximum transmission wavelengths, and the remaining transmission loss is caused by the film bulk properties like absorption and scattering. Comparison between various materials and structures is most easily done by measuring deposited films of various thicknesses and comparing the maximum transmission values. With crystalline film, optical losses are greater at short wavelengths. Thus for example, in the case of applications for human vision it is advantageous to measure losses at about 360 to 440 nm, near the sensitivity limit for the human eye.
In the production of a reliable optical product, low stress or tension in the structure is very important. The thicker a coating is, the more important it is to achieve low internal tension in order to avoid delamination or cracking of the film stack. There are several methods for measuring stress. For example, if one side of a thin glass substrate is coated, tension causes bending of the substrate. The curvature can be measured, and the film stress calculated based on the known properties of the substrate material.
Molecules may diffuse in thin films, during and/or after the deposition, giving rise to various adverse effects. Ideally, thin film materials should be resistant against diffusion. Measurement of the barrier properties is case-dependent. In some cases, adverse effects occur during the deposition process due to incompatibility between chemicals in the film interface, causing increased loss or influence on crystal growth. Some materials may cause problems in the final optical component by causing delamination or added loss due to diffusion of material across the interface.
For the purpose of this text, “oxide” refers to all oxides (for example, titanium oxide, aluminium oxide, tantalum oxide) of various chemical composition, phase and crystalline structure. Correspondingly, where a stoichiometric chemical formula is used, as is common practice in the field, this does not necessarily imply that the layer in question has the corresponding absolute stoichiometric composition. The expression “index” refers to refractive index if not indicated otherwise.
Titanium oxide is known for its high refractive index. Titanium oxide appears in compounds of various compositions, e.g. TiO2, Ti2O3 and Ti3O5. Three crystal forms appear: Rutile, brookite and anatase. Further, titanium oxide appears in an amorphous form. The properties depend on the crystal form. Phases may be mixed, amorphous and crystalline forms may coexist in the same film. Thus, the properties of different films show great variation owing to differences in the manufacturing process.
Unfortunately, using Atomic Layer Deposition (ALD) technology (see below), amorphous titanium oxide is readily generated only at below about 150° C. deposition temperature. This severely limits its usability in optical designs. A deposition temperature above about 150° C. results in crystalline titanium oxide, which gives rise to optical losses. On the other hand, higher deposition temperatures are desirable for example                to enable the use of titanium oxide in thin film deposited structures together with other materials requiring higher deposition temperatures        to increase titanium oxide film index and density        to adjust the residual stress in titanium oxide film        to increase the thin film adhesion to the substrate.        
In addition to the problems related to the crystal structure, titanium oxide is known to be somewhat weak in holding oxygen. Other materials having an interface with titanium oxide may capture oxygen from the titanium oxide layer. In the case of titanium oxide, this leads to films with increased optical absorption.
In batch applications, growing TiO2 film has a tendency to get influenced by surrounding surfaces. This appears as different film growth rate and possibly varying crystal structure of the TiO2 film, depending on what is the opposing surface (distances less than about 15 . . . 20 mm seem to have an effect). If the surface has different structures and is parallel to the growing TiO2 film surface, it causes effects like a “photograph” on the growing TiO2 film. The origin of this effect is unknown at the moment. Especially pure TiO2 film is very sensitive to effects from the surrounding space. This effect limits the possibility to use TiO2 in batch ALD applications, because it sets limitations to the design of the cassette or other fixturing to hold substrates in place during the batch process.
Obviously, a material based on titanium oxide, but lacking the above-mentioned drawbacks of crystals, reactivity and sensitivity for the surrounding, would be very beneficial.
Atomic Layer Deposition (ALD), originally called Atomic Layer Epitaxy, is a thin film deposition process used for over 20 years. Recently, this technique has gained significant interest in the semiconductor and data storage industries. The films generated by this technique have exceptional characteristics, such as being pinhole free and possessing excellent uniformity and step coverage even in structures with a very high aspect ratio. The ALD technique is also well suited for precise tailoring of material compositions and very thin films (<1 nm).
To grow films by means of the ALD technique, substrates are placed in a reaction chamber, where process conditions, including temperature and pressure, are adjusted to meet the requirements of the process chemistry and the substrate materials. Typically, the temperature is in the range 20 to 600° C. and the pressure in the range 1 to 1000 Pascal. Once the substrate reaches a stable temperature and pressure, a first precursor vapor is directed over the substrates. Some of this vapor chemisorbs on the surface, resulting in a one monolayer thick film. In true ALD, the vapor will not attach to itself and this process is therefore self-limiting. A purge gas is introduced to remove any excess of the first vapor and any volatile reaction products. Subsequently, a second precursor vapor is introduced which reacts with the monolayer of the first chemisorbed vapor. Finally the purge gas is introduced again to remove any excess of the second vapor as well as any volatile reaction products. This completes one cycle. This procedure is repeated until the desired film thickness is achieved. A key to successful ALD growth is to have the correct precursor vapors alternately pulsed into the reaction chamber without overlap. Another prerequisite in the ALD process is that each starting material is available in sufficient concentration for thin film formation over the whole substrate surface area and no extensive precursor decomposition takes place.
The principles of ALD type processes have been presented e.g. by T. Suntola in the Handbook of Crystal Growth 3, Thin Films and Epitaxy, Part B: Growth Mechanisms and Dynamics, Chapter 14, Atomic Layer Epitaxy, pp. 601-663, Elsevier Science B.V. 1994. A summary of the ALD technique can be found in Mikko Ritala and Markku Leskelä, Atomic Layer Deposition, Handbook of Thin Film Materials, H. S. Nalwa, Ed., Academic Press, San Diego (2001), Vol. 1, Chapter 2.
Thin layers employing alternating layers of titanium oxide and aluminium oxide have been disclosed in U.S. Pat. No. 4,486,487 by Skarp. This patent explains the molecular Al2O3 barrier properties to protect TiO2 against AlCl3 at 500° C. It was estimated that 0.6 nm (6 Å) of Al2O3 eliminated the effects of AlCl3.
Besides the optimization of insulating properties, this patent also discloses refractive index optimization of this TiO2/AlO3 combination film for use in an electroluminescent display structure.
In U.S. Pat. No. 5,314,759 to Härkönen, Härkönen and Törnqvist, a multilayer phosphor layer system for electroluminescent displays is disclosed. The use of a few atomic layers (<100 Å, 5 to 50 Å, preferably <10 Å) of Al2O3 and other oxides and mixed materials is disclosed as a matching layer to:                Match different crystal structures of the different layer materials on both sides of the matching layer.        Act as a chemical buffer layer to prevent chemical reactions and diffusion.        Equalize stresses caused by differences in the crystal lattice parameters and thermal expansion characteristics.        
In the thesis of M. Ritala (“Atomic Layer Epitaxy growth of Titanium, Zirconium and Hafnium dioxide thin films”, Helsinki, Finland 1994, ISBN 951-41-0755-1), it is mentioned that when TiO2 films having a thickness below 200 nm were deposited on an amorphous substrate, the films were essentially amorphous while those deposited on crystalline substrates were more crystalline. Films deposited using alkoxides resulted in films possessing a more crystalline structure than those grown from TiCl4. Ritala mentions unpublished work by Lindfors, where light scattering from TiO2 films was effectively reduced by incorporating an intermediate Al2O3 layer a few nm thick into them. Incorporation of intermediate Al2O3 layers into TiO2 films reduced the crystallinity. A test series of samples consisting 20 TiO2-Al2O3 film pairs was prepared. The number of ALD cycles for TiO2 between Al2O3 layers was between 900 and 990. The thickness of the Al2O3 layers was 10 to 100 ALD cycles. The deposition temperature was about 500° C., and the growth rate for both Al2O3 and TiO2 was about 0.5 Å/ALD-cycle. Al2O3 was made using AlCl3 and H2O as precursors. TiO2 used TiCl4 and H2O as precursors.
In Ritala's work, surface roughness was measured due to the concern that light scattering (loss) becomes stronger with increasing surface roughness. It is specifically mentioned that 10 cycles of Al2O3 layer had no significant effect on the surface morphology. Thicker Al2O3 layers progressively decreased the surface roughness. It was observed, that transmission increased (losses were reduced) by increasing the intermediate Al2O3 layer thickness until a saturation level was reached at about 75 cycles of Al2O3. This corresponded to about 30 . . . 40 Å.
In U.S. Pat. No. 6,388,378 by Törnqvist and Pitkänen, an insulative film for thin film structures is described. This patent describes an electrically insulating TiO2 and Al2O3-containing film, optimized to resist cracking on alkali metal-free glass. It is pointed out that a high TiO2/Al2O3 content causes cracking.
EP 1 229 356 (Dickey, Long, Törnqvist) is directed to methods and apparatus for the production of optical filters. The methods include alternating exposures of a surface of a substrate to two or more precursors that combine to form a sublayer on the surface. A measurement light flux is provided to measure an optical property of the sublayer or an assemblage of sublayers. Based on the measurement, the number of sublayers is selected to produce an optical filter, such as a Fabry-Perot filter, having predetermined properties.
According to Japanese patent application 200017607, vacuum evaporation and sputtering have been used to prepare optical coatings. However, in comparison to these, ALD films generated have exceptional characteristics, such as being pinhole free and possessing excellent uniformity and step coverage even in structures with a very high aspect ratio. The ALD technique is also better suited for precise tailoring of material compositions and very thin films, and is suited for cost-effective automated batch processing.
Multilayer structures made using ALD technology have been under active research and that work has resulted in several patents. However, over the past 10 years that work has been mainly devoted to semiconductor dielectrics for transistor gate oxide and memory cell applications. These publications describe various ways to increase electrical permittivity, decrease leakage current and increase breakdown strength and related lifespan, or reliability issues. The total thickness of films for use in the semiconductor field is very small compared with films used for optical applications, and typically the publications related to semiconductor applications do not deal with stress or optical properties.